A patent document 1 discloses an art to accurately judge a state of a road surface, and to perform a hydraulic control of a clamping force of a belt type continuously variable transmission which is appropriate for the actual state of the road surface. In particular, a band-pass filter processing is performed to a detection value of a rotation speed of a driving wheel. A value obtained by that processing is totally-integrated with time. A road surface state is judged based on that time total integrated value. When it is judged that the road surface state is a rough road, a clamping force is increased to be greater than that when a smooth road is judged.
However, for accurately judging the state of the road surface, the band-pass filter processing is performed to the detection value, and the total integration is performed with time. Accordingly, it takes time to judge. The control to increase the clamping force does not make it in time even when the rough road is judged. Consequently, a belt slippage may be generated. For example, on a road surface whose a road surface frictional coefficient (hereinafter, described as μ) is spotted (dappled), the driving wheels are slipped on a low μ portion, and immediately then gripped on a high μ portion, so that a torque inputted to the continuously variable transmission is increased. When it takes the time to judge the rough road on this road surface, the control to increase the clamping force does not make it in time due to the response delay of the hydraulic pressure. The slippage is generated between the belt and the pulleys.